Stinky isn't a real Goblin he is an anthromorphicisation of the concept of a Goblin, the storytale Goblin and embodies Goblin nature as seen in literature - the storybook villain but without being evil if that makes sense. As such he has many supernatural abilities taken to extremes. I think the blue fire is just a symptom of his supernatural attributes and a clue to the reader that he isn't "real" . As for Vimes, I doubt he will be main character in many more books, he is very powerful, kudos to Pterry for making such good reads with characters with GodSue like Powers as Vimes is behind the stakes here with Vetinari and Weatherwax around. That's why there is a protagonist shift in recent books. I'd love a Weatherwax-Vimes crossover though.